Modern diesel internal combustion engines operate by compression ignition with direct fuel injection. These engines normally have very low unburned hydrocarbons (HC) and carbon monoxide (CO) emissions. Nitrogen oxide (NOX) and particulate matter (PM) emissions, on the other hand, have remained a challenge to diesel engine combustion and control engineers. The challenge in dealing with NOX and PM emissions is that efforts to reduce NOX generally increase PM emissions, and vice-versa. The relationship between these two exhaust components has been extensively studied and is known in the diesel engine design and manufacturing industry as the NOX/PM tradeoff.
Regardless of the wide acceptance of the trade-off between NOX and PM emissions, it has been known at least since the 1990s that there is an exotic mode of engine operation referred to as low temperature combustion mode by which a diesel engine can be operated with little or no appreciable emission of either NOX or particulate matter.
Low-temperature combustion mode exists beyond the smoke limit. Beginning from a conventional operating mode, increasing the rate of exhaust gas recirculation (EGR) causes a reduction in NOX emissions and an increase in particulate matter emissions according to the usual relationship. As the EGR rate is further increased a point is reached at which the engine emits an excessive amount of smoke. This, the smoke limit, was long considered to be a practical limit on the operating range of a diesel engine. However, as described in U.S. Pat. No. 5,890,360 to Sasaki et al., it has been shown that if the exhaust gas recirculation rate is still further increased smoke is no longer produced. A point is reached at which the engine emits no smoke and both NOX and PM emissions are very low. Operation in this regime beyond the smoke limit is what is meant by low temperature combustion (LTC) mode operation.
The term low-temperature combustion mode reflects the realization that the low NOX and PM emissions are a consequence of the low temperatures at which combustion is occurring. A high degree of exhaust gas recirculation provides a high proportion of inert (with respect to combustion) gases like N2, CO2, and H2O in the combustion chamber. The high proportion of inert gases limits the peak gas temperatures that occur over the course of the combustion process. Low-temperature combustion mode limits peak temperatures to approximately 1800 Kelvin or less, whereas in conventional diesel engine combustion peak temperatures typically are several hundred degrees higher.
It is well known that lower combustion temperatures reduce NOX production, but the reduction in soot formation realized by low temperature combustion mode is more difficult to understand. Soot formation is a complex chemical process involving numerous steps and a large number of chemical reactions. The initial reactions involve the breakdown of the diesel fuel into smaller molecules. In subsequent reactions, the smaller molecules recombine and eventually form very large molecules that make up soot. The explanation for low soot production in low temperature combustion mode is that soot formation process does not proceed beyond the formation of smaller molecules, which are thought of as soot precursors.
While low-temperature combustion mode is laudatory in providing simultaneously low NOX and PM emissions, LTC has severe limitations including low engine efficiency and high emissions of unburned hydrocarbons (HC) and CO. See SAE 2001-01-0655, FIG. 4 (showing brake specific fuel consumption and HC and CO emissions increase rapidly as the engine operation moves into the “smokeless” rich regime) and page 3, last three sentences (stating brake-specific fuel consumption (BSFC) increased to a “serious” extent as the engine entered the fuel rich regime); U.S. Pat. No. 5,890,360, FIG. 2 (showing torque falls as EGR rate increases), FIG. 10 (showing a step increase in torque for a given fuel amount as the engine transitions from LTC mode (region I) to conventional mode (region II)) and col. 11 line 64 to col. 12 line 4 (stating conventional mode is more efficient than LTC mode and requires less fuel).
Low fuel efficiency is reflected by unburned hydrocarbons and CO (unused fuel) in the exhaust and by increased exhaust temperature upon transition to LTC mode (indicating fuel has been used to produce heat energy instead of mechanical energy). See SAE 2001-01-0655, page 4, first column and Appendix A (LTC increases exhaust gas temperature into the 200-250° C. range), page 4; U.S. Pat. No. 7,246,485 to Ohki et al., col. 8 (switching to lean LTC mode provides hydrocarbons that burn in exhaust aftertreatment devices, heating those devices).
Aside from poor fuel efficiency, LTC has a narrow operating envelope. See SAE 2001-01-0655, page 4, col. 1 (explaining that the large amount of EGR limited the operable range to idle and low load); U.S. Pat. No. 5,890,360 col. 9, lines 24-28 (stating LTC is only possible at low load), FIG. 7 (showing the region I in which LTC can be performed and the region II in which conventional combustion is used), and col. 2, lines 38-44 (stating that LTC combustion does not always occur and that it is necessary to determine when to perform LTC); U.S. Pat. No. 6,763,799 to Ito et al. col. 1, lines 36-45 (stating that normal combustion is required to provide drivability except at idle and low load). It is difficult to stably maintain low temperature combustion. See U.S. Pat. No. 6,763,799 col. 1, lines 51-62.
Because of the various disadvantages described above, LTC has been suggested as an option only for special circumstances such as low power and idle operation. See U.S. Pat. No. 5,890,360. When little or no power is required, low fuel efficiency is not a great issue. Also, the low-power regime permits the high degree of EGR required to achieve LTC in the prior art.
U.S. Pat. No. 6,131,388 to Sasaki et al. and U.S. Pat. No. 7,246,485 propose LTC mode to heat exhaust aftertreatment devices at low load and idle conditions by taking advantage of the increase in exhaust temperature and by combustion of the residual hydrocarbons in the exhaust. While diesel exhaust temperatures can reach 500° C., at idle they drop into the 100-150° C. range. LTC raises the exhaust temperatures into the 150-250° C. range. The unburned hydrocarbons and CO provided by LTC operation can be combusted in exhaust aftertreatment devices having oxidation catalysts to further increase temperatures.
In LTC mode, rich engine operation is also possible. U.S. Pat. No. 6,131,388 describes rich engine operation as part of a process for heating exhaust aftertreatment devices. This process involves brief periods of rich LTC engine operation. The hydrocarbons produced by the engine during these rich periods are stored temporarily in an exhaust aftertreatment device having a hydrocarbon storage ability. Following a brief period of rich operation, the engine is operated lean. Oxygen made available by lean engine operation allows combustion of the stored hydrocarbons producing large amounts of heat within the exhaust line. By alternating between rich LTC operation and conventional lean engine operation, exhaust aftertreatment devices can be maintained in the 350-550° C. range. See U.S. Pat. No. 6,131,388.